Note: Descriptions are shown in the official language in which they were submitted.
~53glO
The pre~ent invention relates to the pxoduction
of metal carbides and metals from ores and alloys, in
particular from low grade ores.
This application is a division of copending
Canadian application Serial No. 178,221 filed August 7,
1973.
In the production of metals from ores, a smelting
procedure using coke and a flux generally is practised,
with liquid metal being trapped at intervals. These pro- ;
cedures require the use of high grade ores and high tempera-
tures, and tend to emit noxious gases and particulate matter.
A particular metal récovered in this manner is -~
chromium. Present practice requires the use of an ore
having a chromium to iron ratio of at least 3 to 1 and
attempts to use low grade chromite ores, of varying composi-
tion and having chromium to iron ratios typically 1 to 2:1,
more typically 1.2 to 1.~5:1, in smelting procedur~ have been
unsu~cessful. Considerable effort has been expended in
developing beneficlation technlques for use with such low
grade ores to increase the chromium to iron ratio to at least
3:1 prior to conventional smelting techniques. However, such
procedures have not proved commercially successful.
In addition, high carbon ferrochrome alloys are
used in steeI maklng1 containing typically 55~to 66~ Cr, 7 to 8%
C and the balance Pe, and conventionally are~formed from high
grade~ chromite ore using carbon as a reducing agent.~ Low
car~bon ferrochromlum ;lS produced ~rom high gradè ore and
silicon chromlum alloys as~reducing agent. Low carbon con~
taining~sponge~chrome~materials have not been prépaxed however.
~: i : : : : : . :
30~ The present invention seeks to overcome the above
defects of the prior~ art, making it possible for the first
~S35~
time to recover substantially pure chromium metal Erom
chromite ores and concentrates without smelting and in par-
ticular to recover pure chromium metal from low grade chro-
mite ores. The present invention also makes it possible for
the first time to form a substantially pure chromium carbide ~''
for use as a steel additive from chromi~e ores.
'In the process of the present invention, a sodium ;~chromate free from iron and other ore metal values is formed
from a chromite ore and the sodium chromate is formed into a
chromium carbide or into chromium metal by a solid state re- ~
action. By the use of solid state reaction, lower temperatures '''
are re~uired and the process is essentially non-polluting.
While the invention will be described with particular
re~erence to chromium ores, the invention also is applicable
to the recovery of other metallic carbides and me~als from
ores containlng the metal'values. In particular, the invent- ~'
ion is applicable to metal oxides which are convertible into
bater soluble compounds'on reack1on with alkali, for example, ;~
oxides of vanadium, niobium, tantalum, molybdenum, tungsten
and manganese.
; ~ ~ The production of a pure sodium chromate from low
grade chromite ores has been suggested heretofore, such as
in ~'Utlllzation of Low Grade Domestic Chromate", by K.W.Downes
, .
et al, Department of Mines and Technical Surveys, Canada,
' ~ Mines Branch, Memorandum ~o.116, published 1951, ' ''
EX~r~C~ on of Chromlum from~Egyptian Chromium Ores'l by ~ ~
. . .
M.K.Hussein et al, Canadian Metallurgical Quarterly, Vol.II
~-No. 3, (19'72) p. 4~1 to 48g, and "Chemistry of Chromium
' and its-Compounds", Marvin J. Udy, Reinhold Pu~lishing
Corporation, New York, 1956, especially pages 265 to 273.
- :
':
- 3 -
'
~LCI S39~L~
The chromite ore, ~ypically a low grade chromite
ore, or a concentrate thereof is roasted under oxidizing
conditions with alkali salts, typically sodium carbonate.
Lime commonly also is incorporated in the roasted mixture to
accelerate the oxidation procedure and to form insoluble
compounds with part of the silica and alumina values of the
ore. In this way, the chromium values are converted to
soluble chromate. The iron values of the ore are not
solubilized in this procedure and hence on leaching of the
-roasted material with water, the solubilized chromium values
are separated from the undissolved solid which contains
the iron values, other unsolubilized values of the ore and
insoluble compoun~s.
The alkali roast causes solubilization of at
least some of the silica and alumina values în the ore and
hence the aqueous solution resulting from the leach contains
dissolved quantities of sodium silicate and sodium aluminate.
The pH of the soIution is adjusted to a more acid
value to cause precipitation of alumina and silica which
are separated from the aqueous phase~ which now is substan
tially free from dissolved alumina and silica values. The
pH ma~ be adjusted using su1phuric acid or carbon dioxide.
It is preferred to use carbon dioxide since the alkali
metal values of the silicate and aluminate remain as
carbonate, which may be recovered later and recycled. When
sulphuric acid is used, sodium sulphate remains ln solution,
which may be separated and sold i~ desired.
The aqueous solution rema}ning a~ter separation
of silica and alumina contains dissolv~d quantitiles of a
~ sodium chromate. The oxm of the sodium chromat~ depends
on the pH resulting from the acidification. At pH's about 6,
,
_ 4 -
~53~
the sodium chromate is Na2CrO~ whereas a~ ~H' 9 about 3, the
sod.ium chromate is sodium dichromate Na2Cr207. It is
preferred to have the sodium chromate in the dichromate
form since in this form it is more soluble than Na2CrO4.
The aqueous solution is evaporated to dryness to
form a sodium chromate which is contaminated substantially
only with sodium carbonate, where carbon dioxide has been
used to adjust the pH, or with sodium sulphate where sulphuric
acid is used to adjust the.pH. Any solid material
contaminant precipitating from the solution during evaporation
may be separated.as impurity. ~ -
The solid chromate is free from iron or other
ore metal comtami.nants and is substantially pure except
or the presence of excess alkali values.
The solid sodlum chromate, preferably in the
sodium dichromate form, is reacted in the solid state with
carbon to form chromic oxide and sodium carbonate. The : .
sodium carbonate formed in this way, together with sodium
~ carbonate pxesent in the sodium chromate, may be recycled
to the solubilization s-tepO The reaction is represented :
by the following equation:
: Na2~r2O7 + 2C Cr23 Na2C3 CO tl)
The sodium carbonate may be separated from the chromic oxide
,
by water leaching and crystallization. In addition~ dilute
: acld may be used to remove traces of soda ash moxe expe-
. ` diently. ~ .
: ~ : Since sodium carbonate is formed in this reaction
it is~-pxe~erred~that the-sodium chromate reac~eid i~ln~
~L~ . ~ admixture with sodium carbonate, achieved.by util:ization
~30 of carbon.dioxide to acidify the leach liquor, so ~hat--
separation of the materials leached from the chromic ~xide
.
.: _ 5 _ :
'
. .. . ...... ..
~053~
is unnecessary prior to recycle of sodium -arbonate.
The chromic oxide is subjected to a solid state
reduction to chromium carbide, in accordance with the
equation:
2Cr O ~ 7C ~ Cr C + 6CO - (21
The chromium carbide Cr4C is sometimes written,
perhaps more accurately, as Cr23C6. In the present
- application the former nomenclature will be used. This
~ chromium carbide is the member of a family of chromium ~
carbides, in d uding Cr4C, Cr7C3 and Cr3C2, which has the
lowest carbon content and hence is the most acceptable
product as a steel making additive where low carbon
additions are xequired.
In order to promote interaction between the
oxide and the carbon in the solid state and hence increase
the efficiency of the reaction, it is preferred khat the
~ components are in intimately mixed finely di~ided ~orm and
,: : :
agg~rated.'Preferably this is a~ueved b~ s1urrying poKders of the
materials together, preferably in -300 mesh particle size,
`20 ~ more preferably -40U mesh particle size, followed by dewarter-
ing of the slurry and paxtial drying of the mixture prior to
agglomeration.
- The temperature employed in the reduction reaction,
equation (2)), may vary over a wide range, typically from
about~102'5C to 1425C or h1gher. Over this temperature~
'range~ thé partial~pressure of carbon~monoxide formed;varies
; widely from about 0.09 atmosphe~es at 1025G to abou~ 21~0
atmosphere~s at~ 1425,C_;and,~dictates,to some ex~en,t ~h,e~,t,ec,h~
ni~ue~employed to remove the carbon monoxide. Removal of
30 ~ ~ the ca`rbon,m,ono,x"id,e,rom the reaction vessel pr,omo~es the ' ~'
reduction rsaction. ~t~low partial pressures Oe carbon
:
.
~ 539~LO
monoxide below about 0.1 atmospheres, experienced at the low
end o~ the temperature range, vacuum may be used to remove
the carbon monoxide~ At higher partial pressures above about
O.1 atmospheres, the carbon monoxide may be flushed from the
reaction vessel with an inert gas stream, such as argo~.
It is preferred to flush the carbon monoxide
from the reaction vessel by an inert gas stream, and hence
the higher temperatures are used, generally above about
1200C.
~he reduction is continued until substantially
complete reaction is achieved, generally involving a time
span of about 2 to 4 hours for quantities of the order of~ ~ -
l lb.
The precise for~ of the chromium car~ide depends
on the quantity of carbon used and the reaction temperature,
since the lower temperatures promote the formation of lower -- !
aarbon content carbides. Preferably, the amount of carbon
employed corresponds to the stoichiometry of equation 2,
.
although other carbides, such as Cr3C2 and Cr7C3 may be
~ formed at will by use of the appropriate quantity of aarbon
and temperature range.
The temperature ranges ~or the formation of other
metallic carbides by similar solid state reduction of metal
oxide may vary from the range recited above for the formation
o~ manganese carbide in accordance with the equation:
7 MnO ~lOC ~ Mn7C3 ~ 7C0 - (3)
th~r~dùctk~ may be carried ou~ conveniently in a rotary
~ . .
klln at a temperature in the range of about 1125oc o;abQut;
1325~C, with the carbon monoxide being removed using an
` ;30~ inert~gas~stream~since the-partial pressure of carbon~
monoxide increases from about O.L atmospheres at lL25C
to about 2.1 atmospheres at 1325~C.
.
.:
:
~ 539~
The chromium carbide may be reacted in a solid
state reaction with chromium oxide to form sponge chromiurl
in substantially pure form in accordance with the equation:
3Cr4C ~ Cr2O3 14Cr + 3CO - (4)
The chromic oxide used to achieve this reaction
is that formed from the sodium chromate and hence, in a
preferred embodiment of the invention, the chromic oxide,
which is formed by solid state reduction of the chromate,
is diviaed into two portions, one portion being reduced to
chromium carbide while the other is mixed with the chromium
carbide, the mixture being reacted to form chromium metal.
The division of the chromic oxide into these two portions
preferably is made in accordance with the stoichiometry
of the above equation 4.
The reaction of the chromium carbide with the
chromlum oxide may be carried out~under a wide range of
conditivns. As in the case o~ the solid state reaction
between the oxide and the carbon~to form the carbide, the
reactants preferably are inely divided and intimately
mixed and agglomerated, and the technique described above
to produce the intimate admi~'ture of chromium oxide and
carbon may be used to achieve this intimate admixture.
; ~ The temperature used for the solid state reaction
depends on a number of fàctors, such as the carbide
involved~and the~technigue~to be used for the removal of
the ca~bon monoxide. Typically, a temperature in the range
:, :
a~o~ 1300 to about 1750C or higher may be used.
In the~temperature range where partIal pressures of
- carbon monoxide lower than 0.1 atmo~pheres result, a vacuum
30~ mày b`b used to remove ~ thé c~rbon monoxide from the reaction
ve~sel as it is formedO Where, however, the partial pressure
.... . ~ . . .... . .. ~ . . . .. .. . . . . ... . . . . . . . .
- ~539~
of carbon monoxlde exceeds about 0.1 atmospheres, then pre-
ferably the carbon monoxide is removed using an inert gas
stream, such as argon.
For the reaction of Cr4C with Cr2O3 in accordance
with equation 4, the partial pressure of carbon monoxide at
1525C is about 0.1 atmospheres. Hence, the reaction tempera-
ture preferably exceeds about lS25C and the carbon monoxide
may be removed from the vessel, conveniently a rotary kiln,
using an inert gas stream.
Higher temperatures result in shorter reaction
times, and hence are preferred. However, the evaporation
~f chxomium metal may become significant, typically above
about 1700C~ and to reduce losses of product, a balance
of reaction time with evaporative loss generally is chosen.
It is preferred, especially where highly volatile
metals, such as manganese, are employed to position a
condensation surface adjacent the reactant mixture in order
to condense any metallic vapour evolved during the reaction
and hence eliminate the loss of product due to evaporation.
20~ - The preferred temperatures may vary from carbide
to carbide and from metal to metal. For example, where
manganese carbide and mangane.se oxide react to form
manganese metal, in accordance with the equation:
Mn7~3 ~ 3 MnO ~ lOMn ~ 3CO
the partial pressure of carbon monoxide is about 3.3 x 10 4
atmosph res at 1100C and about 0.3 atmo~pheres at 1625~.
While the reaction may be carried out at temperatures down
to about 1100C, vacuum usually is required to remove the
carbon~monoxide formed at temperatures below about~1350C
;30~ since the partia1 pressure of carbon monoxide at about 1350C
: i5 O. 01 atmospheres.
'~'
9 - :
,
~(~53~
The reaction time required to produce the metal
depends ~:o a large degree on the temperature employed as
well as particle size and uniformity of mixing, and tends
to be short.
The reaction time required to produce the metal de~
pends to a large degree on the temperature employed as well as
particl~ size and uni~ormity of mixing, and tends to be short.
The formation of Cr4C is preferred when chromium
metal is to be formed in accordance with this.invention since
10~ less carbon is required to ~orm the.carbide from the oxide and
less carbon monoxide requires to be removed in the formation
of the metal than in the case when other chromium carbides
are formed.
In some instances, it may be possible to convert
the chromium oxide directly to the metal by first carburizing
a portion only o~ the chromium oxide with the stoichiometric
amount o carbon required to form completely a chromium metal, ~ .:
. so that a mixture of chromic oxide and Cr4C i.s obtained.
: ~ Thls mixture then may be reacted further at higher tempera~
2Q ture to yield chromium metal.
. .
In the latter procedure, therefore, the chromic
: ~ oxide is mixed with the required a~lount of carbon and heated .-
at the required elevated temperature to ~orm the carbide-
: :: oxide mixture. ~The mixture~then is heated to ~he re~uired
higher te~perature to form the chromium metal.
Another alte~native procedure involves direct solid
: . :
: state reduction:of the oxide to the metal by using suf~i~ient ~.
: car~on to-form~carbon monoxide in accordance with the
: : :equation~
- .
~ 30 ~ ~ ~: . Cr203~+ 3C ---~2Cr ~ 3CO
. ~.' .
,,, ~ 1 0 - '
~Q~3~0
This reaction may be carried out ak high temperatures,
typically 1300 to 1700 C, bu-t the volume of carbon monoxide
evolved per unit weight of chromium produced is greater than
that evolved when the required carbon is supplied in the form
of Cr4C or another carbide of chromium.
The invention is described further, by way of
illustration, with reference to the accompanying drawing,
which i~s a schematic flow shee-t of one embodiment of the
present invention.
Referring to the drawing, a chromite ore concen-
trate, which first may be pelletized, is fed by line 10 ~o
a roaster 12. The chromite ore concentrate may be a
concentrated low grade chromite ore, the low grade ore
typically containing 15 to 26~ Cx2O3 and the concentrated
ore typically about 38 to 45% Cr2O3~ with a Cr.Fe ratio in
concentrated ore of about 1.5. The ore is roasted in the
- roaster 12 of any convenient construction, such as, a rotary :
klln, under any convenient conditions in admixture with
. ~ sodium carbonate fed by line 14 to the xoaster 12~ The
roasting lS carried out in the.presence of molecular oxygen
which may be provided by air fed by line 16.
The roasting is carried out under conditions to
oxidize substantially all the chrom:ium values of the ore to
: ssdium.chromate, typically by roasting at about 850QC for
about 2 hours. ~
The~quantity of sodium carbonate added by line
14 to the roaster 12 is generally in excess of the quantity : : :
- :
: required to solubilize all the solubilizable mate.rials o~ : .
the ore concentrate, including the chromium, aluminum and
30~ silicon values of the conc`en~rate. This excess is us'ed-to
: ensure that the majority,~ and preferably subs.tantially all,
1 1
~' .
539~
of the chromium vdlues are converted to sodium chromate.
The roasted mixture, after cooling, quenching
and crushing in any desired manner, is forwarded by line
18 to a leacher 20 wherein the roasted mixture is conkacted
with water fed by line 22 to dissolve soluhle materials Erom
the roasted mixture. The insoluble gan~ue components,
including the iron oxide values of the original concentrate
are separated from the resulting leach liquor solution and
: . removed from the leacher by line 24.
The leach liquor contains dissolved sodi~n salts,
including the chromate, aluminate, silicate and carbonate.
The carbonate is present due to the use of excess sodium
carbonate in the roasting step. However, since substantially
no iron values are solubilized during the roasting step,
substantially complete separation of the chromium values
from the iron values of the ore concentrate is achieved.
The leach llquor then is acidified in reactor 26
by bubbling carbon dioxide therethrough fed by line 28.
. .
Preferably, the leach liquor has a temperature of about 5
~;2~0 ~ to 10C during the acidifying and the carbon dioxide is
~;~- present as an~atmosphere thereof at an elevated pressu~e.
The acidification o the leach liquor achieved in this way
.~
oauses precipitation of alumina~ ana silica from the leaoh
liquor with consequen~-formation o~ sodium carbonate. ~he
precipi~ated alumina and~ SiliGa are separated from~the~mother
liquor and removed by~line~30. The acidifica~ion may be
continued to a p~of~about~3 so that the chromAte ions~ are
converted to dichromate ions~-the solubility o* sodi ~ ~
dichromate~be~ing considera~ly greater than that of sodium
30 ~ hromateO The leach liquor may be maintained at substantially
normal temperatures such as 20 to 25C during the bubbling
:
~ 12 -
.
.
~53~
of the carbon dioxide therethrough.
The aqueous solution of sodium carbonate and
sodium dichromate resulting from the ac:idifier 26 is
evaporated to dryness in an evaporator :32, the evaporated
water being collected by line 34. Prior to commencement
of crystallization of sodium carbonate or sodium dichromate
during the evaporation, any solid deposited material may
be discarded as impurity. The resulting solid, consisting '.
of a mixture of sodium dichromate and sodium carbonate is -.
10. recovered in line 36. , .:
The solid mixture then is mixed in a kiln 38 with ,.. :''
carbon fed by line 40 and is heated at an elevated temperature
to reduce the sodium dichromate to chromium'sesquioxide and ..
sodium carbonate in the solid state. If desired, the intimate
admixture of sodium dichromate, sodium carbonate and carbon '':
,
may be formed by dispersing the required amount of carbon in ~' .
the aqueous solution of sodlum dlchromate and sodium carbonate ' ~
before crystallization thereof. The reduction o~ the sodium ' ':
: dichromate is carried out typicall~ at a temperature of 850C
until all the chromium values are converted to chromic oxide,
typically in about 30 minutes. ~ ~.;:'
In an altexnative procedure, the quantity of carbon
added may be sufficient and the conditions such as to reduce
'the~sodiu~ dichromate to chromium carbide. : '~
The solid mixture of chromic oxide and sodium
carbonate or chromium carbide and sodium carbonate, as the
case may be, after cooling, if desired, is passed by line 42
'o:a;~leac~er 44 whbrein~e'so!~ium'.~arbonate v`alues are'';~
. .- -leached-,.by water fed by.line..46 fr,om the mixture to leave,.....
,3~. -.. .substantially pure ~hromium oxi~e: ~rlchromium carbide. Wash-
, , ~
ing wi~h dilute acid may be used to remove :residual sodium
:
~ 13' - :
~053~
carbonate and thereby further purify the chromium oxide pro-
duct. It has previously been suggested th~t chro~ium oxide be
formed from low grade ores in this manner, but the chromic
oxide formed in this manner has been used to enrich low grade
ores in order to increase the chromium-to-iron ratio of such
ores to a level of at least 3:1 or use in the formation of
ferrochromium alloys by smelting.
The aqueous solution of sodium carbonate resulting
from the leaching in the leacher 44 is removed by line 48.
lQ The sodium carbonate values are recovered therefrom by
~rystallizatlon and the solid sodium carbonate preferably
is recycled to the sodium carbonate feed in line 14 as at
least part thereofO Under ideal conditions, the quantity
of sodium carbonate recovered from the solution in line 48
is substantially the same as that fed to the ore concentrate
by line 14. However, losses of sodium carbonate values may i~
occur and hence it may be necessary to supplement the
recycled sodium carbonate material with additional sodium
carbonate by line 50 to pro~ide the required amount in line
I4.
~ The chromic oxide resulting from the leacher 44
in line 52 is split into two streams in lines 54 and 56.
The proportion o~ the chromic oxide in line 56 is fed to a
kiln 58 wherein the oxide is carburized in intimate admixture
~: :
with carbon fed by line 60 to a chromium carbide. Preferably,
the quantity of~carbon fed by line 60 is stoichiometrically
equivalent to that required to form Cr4C and the carbon
and the chromium oxide are intimately mixcd and agglomerated.
. .",~ ,.c~ J~ t: ~
~ The resulting chromium carbide, or chromium carbide
.: .. .... ... .. . ., .. i. :.: . . ~ ; . , . ~ .................... .. ... ...
30 ; resulting from reduction of sodium dichromate, if that pro-
cedure is used, is fed by line 64 to a reactor 66 after
crushing, if required~ wherein it is intimately mixecl,
l~S39~
ag~lomerated and reacted in admixture with the remainder of
the chromium oxide in line 54 to form sponge chromium having
a low carbon content which is collectecl by line 68, the car-
bon monoxide formed in the process being removed by line 70
by the application of a reduced pressure to the reactor 66
or by the passage of an inert gas, such as, argon, through
the reactor 66. ,
The division of the chromium oxide in line 54 is
made on the basis of the stoichiometry requirements for the
reaction in the reactor 66 to form the chromium metal
without an excess of either reactant left.
The carbon monoxide formed in the kiln 58 and the
reactor 66 and recovered therefrom by lines 62 and 70 may be
and preferably is~ used to provide a reducing atmosphere for ~'
the kiln 38. '
The invention is illustr~ted by the follo~ing
Examples:
Exam~le 1
.
This example illustrates the recovery of
' ~ chromium values as sodium chromate from low grade chromite
ores.
~ Bird River chromite ore concentrate analyzing
26.9% Cr (39.3g Cr2O~) and 22.4~total Fe was sizea to 90
-200 mesh and was mixed with soda ash'and lime similarly
sized to -200 m,esh~' m e mixture was pelletized to approx~
.
imately 1/4" diameter pellets and oharged to a kiln through
which air flowed. 'Thè pel~lets were formed from a total
'weigh~---o~3-8-8-g.-' chromite~on'cent~-at~-, 311`g.---soda àsh, ~'~- '' '
4.3 g. lime and 173.1 g. wa~er.
- - 30 ~ The pellets~were heated for 2 hours u~ to the
~ roasting tempPrature and were roasted at an average ~
:: ~ . ~ .
15 ~ ,
:
, . , ~ :
539~
temperature of about 855C for about 4 hours while about 18.4
SCFH of air passed through the kiln.
The pellets were allowed to cool after roasting
and were quenched to room temperature. Thereafter, the
pellets were crushed and the fine material obtained was added
to about 2 litres of distilled water maintained at about 95
to 100C. The roast product was slurried in the distilled
water and was leached in three stages each of two hours
duration. After each of the first two stage~, the slurry
was filtered and the residue was placed in a fresh quantity
of hot, distilled water and leaching commenced again. The
residue after the third stage was washed.
The resulting aqueous phase was concentrated and
354.6 g of impure sodium chromate were recovered which was
.
found t~ be substantially iron free. ' '
e impure sodium chromate was dissolved in 465
' ml of distilled water and cooled to 10C. CO2 gas was
passed through a copper cooling coil immersed in a water
bath held at about 5C and then into the sodium chromata
' solutlon. The pH of the solution declined from about 13.2
to 80 9. The solid deposited in this process was filtered
and the `filtrate concentrated to deposit a solid material.
` ~he solid recovered from the filtrate was found
to contain 72.7g of the Cr values of the impure chromate
~ .
and 27.9% C~ L86.9~Na2CrO4). The major impurity was 8.6
NaHC03.
: .
~ ~xample 2
:
This example-illus~trates the formation~of--chromi~
oxide,,~rom sodium chr,omate,produced in~a procçdure siml1~r,~;
30~- - to,that outlined in Example 1. 5, ' ' '~
5 g of sodium chromate crystals containin~ 23.9%
~ 16 ~
~539~
Cr (74.5% Na2CrO4) obtained from Bird River chromite ore
were crushed to 100~ -200 mesh and mixed with 0.29 g of
graphite sized to 100% -400 mesh and contain:ing 95.0%
fixed carbon. The mixture was placed in a crucible in a
Vycor tube which was ~lushed with 0.19 SCFM of nitrogen
for 5 minutes and placed in a furnace preheated to 1000C.
The charge was rapidly heated and held at 1000C for 30
minutes under a flow of nitrogen. The products were cooled
~uickly. After crushing, the 4.5 g of products were washed
in hot water at 60C to remove sodium carbonate and any
residual sodium chromate. Residual sodium car~onate was
removed by washing the product with,l:10 hydrochloric acid.
Any excess carbon was removed by roasting the leached product
in air at 1000C for one hour. The final weight of product
obtained was 1.80 g. This product analyzed 67.3% Cr,
giving a Cr2O3 content of 98.4%.
Example 3
This example -illustrates the formation of chromium
carbides from chromic o*ide~~
]00 g of -400 mesh commercial grade chromic o~ide
98.5% Cr2O3) was dry mixed with 72 g of -400 mesh graphite
~95.0~ fixed carbon). The mixture contained 2 times the
stoichiometric carbon requirement based on ~he reaction: ,
3cr23 -~ 13C - ~ 2Cr3C2 ~ 9C~
'was pelletized with moisture and the pellets were dried.
~ The pellets were fire~ in alumina crucibles in ;
the presence of argon at a flow rate of 0.05 CFM at about
1300C for 2 hours. After`''cooling, 't~e pellets''wer~ cr'ushed'`'~'
,
-and~thelbulk of the exce~s~carbon~ w~s`removed by flotation '~
in water.~~The product was~sub~jected'"`to X-ra~ diffrac~ion ~''
analysis and found to have the approximate composition:
- 17 ~
, - . .. . . . . . ... .
~ 353~
80% Cr3C~, 15% Cr7C3 and 5% graphite. The product
analyzed 72.5% Cr and 14.5% C.
Example 4
The procedure of Example 3 was repeated with
a firing temperature of from 130Q to 1380C for a period
of 3 3/4 hours 320 g of pallets were fired and 218.6 g of
product were obtained.
The product was ground and sareened o~ 100 and
400 mesh screens to separate excess carbon liberated during '
the cxushing.' The ~uantity, composition and X-ray diffraction
analysis for each fraction is reproduced in the following
Table I:
TABLE I
Fraction Wt. Wt.~ ,Wt~%X-ray Diffraction Analysis
(Mbsh) (g~ Cr CCr7C Cr~ Free C
W~.% W~.g Wt.% ~ .g Wt.% Wt.g ,'
+lon 22~g 80,0 11.7 20.0 4.58 80.0 18.3 ~1.0 0.2
100 ~ ~9.7 73.3 23.1 10.0 4.97 80.0 39.8 10.0 5.0
400
-400 145.0 50.0 47.7 5.0 7.25 75.0 108.~ 20.~ 29.0
Example_5 ' ' '
, ~ Example 3 was repeated, with the pellets being '
formed by dry mixing the chromic oxide and graphite, slurrying
the mixture in water and then vigorously stirring the mixture ~ ,
until it became uniform i~ colour. The excess water was ~' -' -
evaporated slowly and ~he resulting mixture pelletized to form
pellets 3/4" to 1" in diameter~ The pellets were dried at
. . . . .
~ 10~C~or 5 hours. The dried pellets were fired in arg~n ''
: :
~ ; - , " for,7,~urs~nd 5~,minutes at temperatur~s ~anging from~ = '~ ',
.
~ 1300~;to 14SqC,~ Only the stoichiometric re~uirement o~ ;,, ,, '~
grapbite ~or the equati,on in Example 3 was ~Ised. ~ , , -
.
: ' .
~ ' ' '
227.6 g of product in the ~orm o~ very hard grey
pellets were obtained. The pellets were crushed and the
powder analyzed for Cr and C subjected lo X-ray diffraction
analysis. The product contained 83.7% ('r and 11.65% C with
a Cr recovery of 97.3%. The X-ray diffraction analysis
indicated that the product was composed of approximately
75% Cr3C2 and 25%Cr7C3.
Example 6
Example 5 was repeated with half the stoichiometric
carbon requirement for the equation in Example 3. The
pellets were fired in argon for 6 hours at temperatures from
1300 to 1400C.
The crushed product was analyzed for Cr and C
and &ubjected to X-ray diffraction analysis r which revealed -
a product containing 75.0~ Cr and 2.49%C and composed of
approximately 70% Cr2O3 and 30% Cr23C6 (Cr~C).
Example 7
This example shows the formation of sponge ;
chromium from chromium carbides.
The -100 ~ 400 mesh fraction obtained in Example
4 was subjected to elutriation in a column of water flowing
upwardly at a vertical linear speed of 4 inches per minuteu
~eavy and light fractions were obtained and 20 g of the
light fraction; containin~ 71.0% Cr and 25.3% C, were mixed
with 19.23 g of commercial grade chromic oxide. The mixture
: .
- was pressed to form a 1" diameter x 1 1/4" hig~ compact
.
~ containing 90% of the stoichiomètric oxygen required to form
.
`''`i"'~h~ h~':'` ''CO ~w~t~-thie carbon content~
~ . , The char~e was placed~in a molybdenum foil
.:
~r~ ~ ~ 30 ~ 1 crucihl~and~eate~-via a molybenum-susceptor under a vacuum
o~ 27 to 30 mm Hg. After 1 hour and 25 minutes, the
~ 19 -- :
.
3Lo~i3~
temperature a-t the top of the compact had reached 2420F
while that at the bottom had reached 2850F. At this
point some vapour was evolved from the compact and droplets
were observed adhering to the inner surface of the molybdenum
foil crucible, and hence the compact was adjudged partially
molten.
The power then was.shut off and after cooling,
31.85 g of product were recovered. The product was crushed
and subjected to wet chemical and X-ray difraction analysis.
The product analyzed 94.6~ Cr, 1.30~ C and contained
approximately 65~ Cr metal, 20% Cr23C6 and the balance
unidentified constituents which were neither normal
chromium carbides nor oxides.
. Exampl~ 8
This example shows the production of sponge -- t; ~ '
chromium directly from chromic oxide.
~ - Flnely divided chromic oxide sized -400 mesh
: and of purity 98%~ was slurried with graphite sized -400 :~
me~h (95~ C) at hiyh speed for about 20 minutes after
which the slurry was partially dried and formed into a
.,, ~, .
: compact. The ~uant~.ty of graphite was stoichiometric to
.: . form car~on monoxide from th~ oxide in accordance with
' ~ - .
the equation~
Cr23 + 3C ~ 2Cr + 3CO
A total weight o~ 21.7 g of material was slurried of which
80.3% was chromic oxide
The compact was placed in an alumina crucible in .
. a -chamber maintained under a-vacuum of 70 cms o~imercur`y.` `
Th~e aompact was h~ated-at a~temperature Qf abou~` ~S00`to ~
3aJ : ~ 1700C un~`ll it exhib:i~ed-a melted appearance a~ter abou~ 15
. .
.: : minutes~ After cooling, it was found that a loss of about
,:
: - 20 - ~
.. .' ,- ,,.
~, . .
~153~
10~ of Cr values had occurred, probably due to vaporization,
and the product was found by X-ray diffxaction analysis to
be 95% sponge chromium and 5~ chromic oxide.
The present invention therefore provides a
process for the production of sponge chromium from low
grade chromite ores in high yield and iII a non~polluting
manner. The invention clearly is applicable to the
production of other metals from low grade ores by the
same procedures.
Modiications are possible within the scope
of the present invention.
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